The present invention relates to apparatus and method for searching a wireless network system for wireless station locations.
When a wireless network system is built, it is necessary to arrange wireless stations such that wireless communications are established between wireless stations and between wireless terminals with a probability higher than a given value. Furthermore, a tradeoff relationship exists between a requirement that each one wireless station provide wide area coverage and a requirement that the interference between wireless stations be reduced. Therefore, in order to suppress the costs, it is necessary to appropriately determine the number of wireless stations and their radio-wave output power levels. It is known that propagation of radio waves between wireless stations and terminals is affected by structural objects and geography around the wireless stations and terminals. In view of these circumstances, discussions have been conducted to improve the accuracy of estimation of the state of radio wave propagation while reducing resources used to calculate the state.
For example, ray tracing algorithms, FDTD (finite-difference time-domain) method, finite element method, and other methods are available as techniques for analyzing radio wave propagation in wireless communications. Any of these methods fundamentally performs approximate calculations of Maxwell's equations. However, the accuracy and time of computation are different among different calculational methods regarding various conditions including the scale of the wireless service area of interest, complexity, and frequency band of radio waves.
A ray tracing algorithm is considered to be a far-field approximation to Maxwell's equations, and is effective where the wavelength is short compared with the size of the structural objects to be computed. A ray tracing algorithm is also known as a geometric optics approximation, and it can be said that electromagnetic waves are approximated by light propagation. Therefore, it is easy to calculate the state of the light propagation path, and only a small amount of computational resources is required. However, there is the disadvantage that the accuracy of estimation of the state of propagation path is deteriorated in cases where the effects of near fields are great.
The FDTD method is an analysis technique for directly solving Maxwell's equations by discretizing them on the time and spatial axes while retaining the differential form. That is, in the FDTD method, an analyzed region is divided into lattice cells. Electric and magnetic fields calculated about one lattice cell are input to an adjacent cell. Thus, electric and magnetic fields are calculated in turn. The FDTD method can improve the accuracy of estimation of the state of propagation by reducing the size of each lattice cell and shortening the time interval. However, there is the disadvantage that the amount of computational resources is huge because the size of the analyzed space is increased and/or because the frequency is in the radio frequency domain.
In the finite-element method, electric field variables or magnetic field variables defined in terms of finite elements are found from Maxwell's equations based on the variation principle such that the energy of the system is minimized. Therefore, the finite-element method can analyze a region by dividing it into lattice cells in the same way as in the FDTD method. Consequently, the finite-element method has the same disadvantage as the FDTD method.
The prior art technique relative to optimization of the locations of base stations is disclosed, for example, in patent document 1. In particular, at each geographical point of wireless cells formed by base stations, the presence or absence of the visibility to every base station existing around is calculated based on geographical and height data. The positions of the base stations are computationally optimized such that a maximum is obtained at a geographical point providing a visibility to at least one base station. Non-patent document 1 discloses another method. Specifically, a propagation path is determined by a ray tracing algorithm. If a structural object for which near fields cannot be neglected is present in the determined path, the structural object is analyzed using an FDTD method while using the state of the propagation path as an incident condition. The state of the output from the structural object is computed. The state of the output is taken as the state of the next propagation path. The next propagation path is determined again using the ray tracing algorithm.
Prior art techniques relevant to the present invention are disclosed, for example, in JP-A-2006-191699 (patent document 1) and by Yongming Huang et al., “A Novel Technique for Indoor Radio Propagation Modeling”, Electromagnetic Compatibility 2002 3rd International Symposium on 21-24 May 2002, pp. 335-338 (non-patent document 1).
However, the propagation analysis technique described in patent document 1 is based on the premise that a far-field approximation holds. Therefore, if a far-field approximation does not hold in spite of the fact that an optimization computation is performed, the presence or absence of a visibility gives rise to large error. This presents the problem that base stations are not arranged appropriately. Especially, in a closed space such as an indoor space, electric power is not easily diffused to remote locations and there are many objects or areas having sizes for which the effects of near fields cannot be neglected. Therefore, there is the problem that large error is produced. In the technique of non-patent document 1, whenever a light ray used in a tray tracing algorithm impinges on an area or structural object for which the effects of near fields cannot be neglected, an analysis is performed on the area using an FDTD method. Consequently, there is the problem that the quickness of the ray tracing algorithm is spoiled. Furthermore, a long computation time is taken to perform analyses using FDTD. Consequently, this technique is not suitable for an optimization computation in which an analysis is performed repetitively while varying the wireless station positions.